Class D audio amplifiers are well-known and widely recognized to provide energy efficient audio drive of various types of loudspeakers by switching a pulse width modulated (PWM) or pulse density modulated (PDM) signal across the loudspeaker load. Class D audio amplifiers typically comprise an H-bridge output stage coupled to the loudspeaker load to apply a pair of oppositely phased or complementary pulse width modulated audio signals across the loudspeaker. An inductor-capacitor (LC) lowpass filter is often inserted between the H-bridge based output stage and the loudspeaker load to suppress carrier wave components of the PWM or PDM output signal. Several types of pulse width modulation have been utilized in prior art class D audio amplifiers. In so-called AD modulation, the pulse width modulated audio signal at each output terminal or node of the H-bridge output stage is switched, or toggled, between two different voltage levels in opposite phase. The two different levels typically correspond to the upper and lower power supply rails, respectively, such as the positive and negative DC supply rails of the class D audio amplifier. In so-called BD modulation, the pulse width modulated signal across the loudspeaker load is alternatingly switched between three voltage levels of which two levels correspond to the above-mentioned upper and lower DC power supply rails while the third voltage level is a zero level that is obtained by simultaneously pulling both sides of the loudspeaker load to one of the DC power supply rails. Both of these modulation schemes generate rather large ripple current in the output inductor of the LC lowpass filter when the class D amplifier is idling which cause significant power losses. This disadvantage has typically been tolerated and controlled to a certain extent by use of relatively large inductors of the LC lowpass filter. However, such large inductors lead to significant increase of costs and size of the class D amplification solution or assembly.
So-called multi-level PWM modulation is an advantageous form of pulse width modulation of class D audio amplifiers and possesses numerous benefits over traditional AD and BD modulation as described in detail by the applicant's PCT publication WO 2012/055968. This kind of multi-level PWM modulation typically involves various types of circuitry to generate one or more intermediate DC supply voltages, for example a mid-supply voltage level, situated between positive and negative DC supply rails or voltages of the class D audio amplifier in question. The one or more intermediate DC supply voltages is/are a low impedance source of a third voltage level, and further voltage levels, of the PWM drive signal applied to the loudspeaker for sound reproduction. An output driver or power stage of the multi-level class D audio amplifier comprises at least four series connected or cascaded controllable semiconductor switches, for example MOSFETs, with at least two intermediate circuit nodes situated where a pair of the controllable semiconductor switches are interconnected. The one or more intermediate DC supply voltages must be sequentially connected and disconnected to at least some of these intermediate circuit nodes to accurately produce the desired multi-level PWM output signal. Accordingly, there is a need in the art for effective control circuitry for sequentially connecting and disconnecting the one or more intermediate DC supply voltages to one or more intermediate circuit nodes of the output driver or power stage of the multi-level class D audio amplifier.